Quantum bits, or qubits, are the building blocks of quantum
computing and physicists have long been working to create ones that
exist in a sold-state system at room temperature. Most existing
systems use complex equipment that tries to trap single atoms or
electrons in a vacuum, which is then cooled close to absolute
zero.

However, the Harvard team led by physics professor Mikhail
Lukin, says it has cracked the problem by using diamonds. The
team took advantage of a pair of impurities in ultra-pure,
lab-grown diamonds to create quantum bits that could store
information for almost two seconds. The work, published in
Science, paves the way towards the construction of a
functional quantum computer.

Lukin explained: "What we've been able to achieve in terms of
control is quite unprecedented. We have a qubit at room temperature
that we can measure with very high efficiency and fidelity. We can
encode data in it, and we can store it for a relatively long time.
We believe this work is limited only by technical issues, so it
looks feasible to increase the life span into the range of hours.
At that point, a host of real-world applications become
possible."

The groundwork for this research started several years ago, when
researchers found that atomic-scale impurities in lab-grown
diamonds, called nitrogen-vacancy centres (NV centres), behave in
the same way as single atoms. Just like atoms, each centre
possesses a spin that can be polarised. Researchers can control the
spin and detect its orientation as it changes over time using
lasers.

NV centres weren't seen as a practical basis for quantum
computing as they could only hold data for about a millionth of a
second. However, Lukin discovered that this was down to another
impurity in the diamond crystal. In early experiments, the team had
used diamonds that contained 99 percent carbon-12 atoms, which have
no spin. The remainder was made up of carbon-13 atoms, which have a
spin at the atom's nucleus. The weak interaction with those spins
was causing the NV centres' short data storage spans. Lukin and his
team sought to turn this interaction to their advantage.

"The nuclear spin of the carbon-13 makes an ideal quantum bit,
because they are very isolated. Because they interact with so few
outside forces, they have relatively long coherence times. Of
course, the same properties that make them ideal qubits also make
them difficult to measure and manipulate," he said.

Rather than trying to measure the carbon atoms' spin, they
sought to use the NV centre to do the job for them. They worked
with British company Element
Six to develop an even more pure diamond crystal that was 99.99
percent carbon-12. They then blitzed the crystal with nitrogen to
create the NV centre, which interacts with the carbon-13 atom. As a
result, the NV centre mirrors the state of the carbon atom, meaning
that researchers can encode a bit of information into the spin of
the atom and then "read" that data by looking at the NV centre.

To make truly useful system, researchers needed to find a way to
take advantage of the atom's quantum properties, i.e. the ability
to be in two states at the same time. Instead of encoding bits of
information as either a zero or a one, as with traditional
computers, quantum computers use atomic-scale quantum mechanics to
allow quantum bits to have both values at once. Because of this,
quantum computers could theoretically perform multiple computations
in parallel, making much more powerful.

In order to do this, the team firstly cut the connection between
the NV centre and the carbon atom. They used "massive amounts" of
laser light to keep the NV centre occupied and preventing it from
interacting with the carbon-13 atom. They then bombarded the
diamond with specific radio frequency pulses to suppress the
interaction between the carbon-13 atom and any other atoms in the
area. By limiting these interactions, the team could extend the
life of the qubit, allowing it to store data for almost two
seconds.

Lukin imagines such a system could be used in applications such
as "quantum cash", a payment system for bank transactions and
credit cards that relies on the coding of quantum bits to thwart
counterfeiters, and quantum networks -- a highly secure
communications method that uses quantum bits to transmit data.

This is not the first time that a sold-state quantum computer
has been built inside a diamond. A combined team from the US and
the Netherlands revealed a similar system that had also been bulit
using imperfections in a diamond in April 2012. In their system, they used the nucleus of a
nitrogen atom to act as the qubit, with an electron from another
imperfection as another qubit. That team used microwave pulses to
switch the direction of the electron spin.